This invention relates to the preparation of an insoluble carboxylic acid from the corresponding alkali metal carboxylate in an aqueous solution. In particular, the invention relates to the use of an electrochemical cell and a double decomposition reaction for the conversion of salts of carboxylic acids into the corresponding carboxylic acids.
The alkali metal carboxylates are usually prepared by the reaction of an amine with formaldehyde, cyanide, and an alkali metal hydroxide. See the U.S. Pat. Nos. 2,387,735 2,407,645 and 2,885,428, which later patent not only described the producton of ethylenediaminetetraacetic acid (EDTA), but typifies the chemistry presently used to isolate the acid product. In said patent, hydrolysis to the free acid is accomplished by reacting the nitrile with a strong mineral acid such as hydrochloric or sulfuric. This results in either the formation of 2 moles of (NH.sub.4).sub.2 SO.sub.4 or 4 moles of NH.sub.4 Cl per mole of EDTA, depending on which acid is used. U.S. Pat. No. 2,407,645 teaches that the presence of alkali metal hydroxide has the advantage of aiding the rapid and complete hydrolysis of the nitrile formed. The presence of alkali metal hydroxide results in the production of an aqueous solution of the sodium salt of ethylenediaminetetraacetic acid. On acidifying to a pH between 0.75 and 2.0, the corresponding free acid precipitates with the simultaneous production of the corresponding inorganic salt. Therefore, in both examples cited, an acidification process is used to isolate the free acid with the simultaneous production of an organic laden inorganic salt stream.
The present invention performs the same isolation without the production of an undesirable inorganic waste stream. The present invention has the added benefit of simultaneously producing an alkali metal hydroxide stream.
An additional embodiment of this invention is the direct addition of ammonia, as a gas or as ammonium hydroxide, to the stream containing the carboxylic acid. NH.sub.3 gas produced in the process may be added directly to the middle stream. The presence of H.sub.2 in the catholyte gas stream has no deleterious effect on the process when that stream is used as an ammonia source. The NH.sub.3 produced in the catholyte may also be added to the anolyte stream, or an outside ammonia source may be used.